The invention relates generally to semiconductor lasers. In particular, the invention relates to mirrors defining the optical cavity of a semiconductor edge-emitting laser.
Semiconductor lasers are being increasingly used as the light source for telecommunication systems utilizing optical fibers as the transmission medium. The laser is positioned to irradiate one end of the fiber, and an electrical signal either directly modulates the laser or controls a modulator through which the laser output passes before it enters the fiber. Optical fiber has the capability of transmitting the optical signal for hundreds of kilometers without regeneration or amplification. An optical detector receives the optical output at the other end of the fiber, and the electrical signal output by the detector corresponds to the modulation signal used at the input end of the fiber.
Semiconductor lasers have several advantages. They operate at relatively high efficiency and are relatively rugged. A waveguide laser produces an optical output that is easily coupled into a single-mode optical fiber. Semiconductor lasers can be fabricated to emit in the two portions of the optical spectrum most favored for the transmission over silica fiber, the 1310 nm band and the 1550 nm band. These two bands are achieved by using semiconductor materials from the AlInGaAs and InGaAsP families grown on InP substrates.
The precise lasing frequency is determined by a number of factors. A typical edge-emitting semiconductor laser 10 is illustrated schematically in FIG. 1. A chip 11 of the semiconductor composition capable of emitting in the desired optical regime includes a p-type layer 12 and an n-type layer 14 separated by an intrinsic active layer 16, which may include one or more quantum wells. The substrate under the lower layer 16 is not explicitly shown. Optical waveguiding means are included to confine light in both the vertical direction and in the horizontal direction perpendicular to the plane of the illustration. Viewed from above, the laser and associated electrode and waveguiding structure appear as a stripe extending between the chip facets. The vertical confinement structure is closely associated with the intrinsic layer 16 and preferably is implemented as a distributed Bragg grating or reflector (DBR) 17 having an optical period P generally corresponding to the desired emission at wavelength xcex0. Two mirrors 18, 20 are formed on the chip facets and define the ends of an optical cavity of length L. The chip facets are usually fabricated by cleaving the crystalline substrate along a cleavage plane so that the facets are mirror smooth and perpendicular. As such, the facets themselves may be sufficient to acts as mirrors, at least on the output side, but additional metal coatings may be deposited. One mirror 18 is made as reflective as possible while the other, output mirror has a small but finite transmissivity to allow output of laser light 22 at wavelength xcex0. Electrodes 24, 26 are contacted to the p-type and n-type layers 12, 14 and a power supply 28 forward biases the p-n junction. For telecommunications, the power supply 28 can be electrically modulated with a data signal, or alternatively a separate optical modulator can receive CW laser light 22 and modulate it according to the data signal.
In an edge-emitting laser, the cavity length L is typically of the order of millimeters or greater and thus much longer than the lasing wavelength xcex0 so that a large number of potential axial waveguide modes 30 exist, as illustrated in the power/gain spectral diagram of FIG. 2. Lasing occurs for a particular mode 30 when, at the wavelength of the mode, the round-trip gain through the optical cavity exceeds the round-trip loss. If this condition is satisfied for more than one mode 30, usually the mode with the largest excess gain draws power away from the less favored modes. An optical gain spectrum 32 in the amplifying portion of the laser is determined primarily by the precise composition of the ternary or higher-order compound semiconductor used to produce the desired optical emission wavelength, and in these materials the gain spectrum 32 is relatively wide. The gain is often expressed in terms of an absorption coefficientxcex1 per unit length with the round-trip gain being equal to 2xcex1L. The gain bandwidth is usually greater than 10 nm and often much greater. The loss spectrum is composed of at least two components. The mirror loss 34 is small and essentially flat if the mirrors are metallic or rely on the semiconductor/air interface. On the other hand, the Bragg transmission spectrum 36 is relatively sharply peaked with a peak position at P determined by the Bragg grating 16. Transmission bandwidths of 0.3 nm are typical. Any additional loss in the waveguide is included in the absorption coefficient a, which in a laser has a sign that denotes power growth. Typically the internal loss is relatively flat over the wavelength bands of interest. The mode 30 that experiences the largest value of gain less loss is the one that lases. The resultant lasing wavelength peak is very narrow.
The narrow laser bandwidth and the control of the emission frequency by the setting of the physical pitch of the feedback grating allows multiple lasers emitting at slightly different wavelengths to be integrated on a single semiconductor substrate. Such a multi-wavelength laser is particularly advantageous for a wavelength-division multiplexing (WDM) telecommunication system in which a single optical fiber carries multiple optical signals which have slightly different wavelengths and which are separately modulated by respective data signals. WDM can multiply the transmission capacity of a fiber by the number of wavelength, which in current systems may be from 4 to 40 or even higher. For the larger number of WDM channels, the WDM wavelengths are separated by approximately 1 nm. Zah describes an example of such an integrated multi-wavelength laser in U.S. Pat. 5,612,968, in which he references fabricational techniques described by Zah et al. in xe2x80x9cMultiwavelength light source with integrated DFB laser array and star coupler for WDM lightwave communications,xe2x80x9d International Journal of High Speed electronics and Systems, vol. 5, no. 1, 1994, and in xe2x80x9cMonolithic integrated multiwavelength laser arrays for WDM lightwave system, xe2x80x9d Optoelectronicsxe2x80x94Devices and Technologies, vol. 9, no. 2, 1994.
However, a useful WDM network includes some amount of optical switching so that an optical fiber may be transmitting and a receiver array may be receiving optical signals originating from multiple transmitting sites. Any overlap of nominally different wavelengths at any point in the network must be avoided so that the wavelengths of the different emitters must be tightly registered. But, supposedly identical laser arrays may be transmitting at somewhat different wavelengths. DBR lasers rely for their emission control on the e-beam writing of the grating, which must be controlled to 0.03 nm for a 240 nm pitch. Such fine lithography imposes a major problem in fabrication and reduces yield below an economically useful level. Furthermore, the semiconductor composition of the waveguide and the Bragg grating introduces a large temperature dependence, with a typical wavelength drift with temperature of 0.1 nm/xc2x0 C. As a result, the temperature must be stabilized at all times and at all transmitting sites. Such control is possible, but expensive and operationally difficult in a distributed commercial environment.
An alternate laser structure is the vertical-cavity surface-emitting laser 40 schematically illustrated in FIG. 3 following the original disclosure by Jewell et al. in U.S. Pat. No. 4,949,350. A tall semiconductor stack having a diameter of about 1 xcexcm is etched from layers epitaxially grown on an n-type substrate 42. From the bottom up, the stack includes an n-type semiconductor lower interface mirror 44, a n-type lower spacer layer 46, an active layer 48 including one or more quantum wells, a p-type upper spacer layer 50, and a p-type semiconductor upper interference mirror 52. A gold layer 54 serves both as a reflector and a contact layer for an upper electrode 56. A lower electrode 56 is contacted to the substrate 42. The interference mirrors 44, 52 comprise a large number of semiconductor layers having a periodic variation in the dielectric constant with the period corresponding to the desired wavelength xcex0 of laser light 22 emitted from the bottom of substrate 42. The mirrors 44, 52 define the ends of an optical cavity of optical length S, which is usually xcex0/2 or an odd multiple thereof. The illustration does not distinguish the optical length from the physical length. The spacers 46, 50 provide the thickness for this length S not provided by the thin active layer 48. The vertical-cavity laser supports usually only a single longitudinal mode. As should be apparent from the disclosure of Chang-Hasnain in U.S. Pat. No. 5,029,176 the lasing wavelength is primarily determined by the cavity length S and not by the spacing in the semiconductor interference mirrors 44, 52. That is, the mirrors transmission spectra are wide enough to encompass significant variations in the cavity length S. The complex semiconductor interference mirrors are required in the illustrated configuration because of the need to inject current through both ends of the laser stack and to extract light through at least one end.
Subsequent work has developed vertical-cavity lasers which emit from the front surface of the chip and utilize an dielectric stack interference mirror on the front surface. See, for example, U.S. Pat. 5,034,958 to Kwon et al.
The vertical-cavity lasers exhibit increased thermal effects because of the isolated geometry of their semiconductor stacks. The lasing wavelength xcex0 is determined by the optical length of the cavity. Both the physical length and the semiconductor dielectric constant, upon both of which the optical length depends, are temperature dependent so fine temperature control is required if a stable emission wavelength is to be achieved.
Edge-emitting solid state lasers are also used as pump sources for erbium-doped fiber amplifiers (EDFAs), which require an intense pumping signal at either 980 or 1480 nm to amplify optical signals in the 1550 nm band. The preferred 980 nm pump signal is produced by AlGaAs or InGaAsP lasers grown on GaAs substrates while the 1480 nm pump signal is produced by AlInGaAs or InGaAsP lasers grown on InP. Both the pump and data signals are coupled into the EDFA and collinearly propagate on it. Conventionally, no wavelength selection is applied to the pump laser, and the gain-minus-loss spectrum is determined by the optical gain spectrum. The lack of mode selection, however, produces a multi-mode emission or jitter between modes of slightly different wavelengths. Although all the pump modes effectively pump the data signals, it has been found that the resultant amplification is unduly noisy so as to increase the noise figure of the EDFA. It is desired to obtain single-mode pump power without the need for complex frequency selection.
Accordingly, it is desired to design a semiconductor laser in which the emission wavelength is stable with temperature and does not require ultra-precise lithography.
The invention may be summarized as a semiconductor edge-emitting laser having dielectric interference filters attached to at least one of the opposed facets of the chip so as to define the optical cavity. The two filters are designed for maximum transmittances at two wavelengths closely bracketing the design emission wavelength. The sum of the transmittances, which represent cavity mirror loss, is doubly peaked with a minimum at the emission wavelength. The much broader gain spectrum is designed to peak at or near the transmittance minimum, whereby lasing occurs at the wavelength of the transmittance minimum.
In a multi-wavelength opto-electronic chip having multiple laser stripes extending in parallel between opposed chip facets, pairs of dielectric interference filters are attached to the opposed ends of each stripe, and the resonant frequencies of the respective pairs are chosen to bracket the frequency at which the respective stripe is intended to lase.